1. Field of the Invention
The present invention relates to an electrode wire for use in a wire electric discharge machining process wherein a workpiece is machined by melting desired portions thereof by electric spark discharge. The invention relates also to a process for preparing such an electrode wire.
FIG. 1 is a schematic illustration showing a general wire electric discharge machining process, wherein a workpiece 1 is formed with a starting hole 2 through which an electrode wire 3 is inserted. The electrode wire 3 is then passed in the direction perpendicular to the workpiece 1 (in the direction shown by the arrow in FIG. 2) while applying an electric voltage between the electrode 3 and the interior wall of the hole 2 to develop spark discharge therebetween so that the workpiece 1 is melted. As the workpiece 1 is moved in the direction perpendicular to the passing direction of the electrode wire 2, the wire 2 traverses through the workpiece 1 along the desired cut line to melt the same, whereby the workpiece 1 is cut to have a desired contour. As shown in FIG. 1, the electrode wire 3 is continuously supplied from a feed reel 4 to pass over guide rollers 5 and taken up by a take-up reel 6. A tension roller 7 is disposed between one of the guide rollers 5 and the take-up reel 6 to adjust the tension applied on the electrode wire 3. Although not shown in the figure, a processing liquid is supplied to the portion which is machined by electric discharge so as to cool the electrode wire 3 and to remove debris.
The electrode wire 3 generally used in such a wire electric discharge machining process is a copper, brass (an alloy typically containing 65% of Cu and 35% of Zn) or zinc-plated brass wire having a diameter generally ranging from 0.05 mm to 0.3 mm. A tungsten or molybdenum wire may be used for special applications.
During the electric discharge machining step, the electrode wire 3 is heated to about 300.degree. C. to be subjected to severe thermal loading while being applied with a tension from the tension roller 7 for maintaining a stable spark discharge for the accurate machining and for the increase in machining speed. It is thus required that the electrode wire 3 should have a high strength at high temperatures, particularly high tensile strength at high temperatures. Although a copper wire may be easily elongated to form a small diameter electrode wire, it is inferior in tensile strength and apt to break to reduce the efficiency of electric discharge machining operation. A brass wire has a tensile strength as high as about two times as that of a copper wire at room temperature, but the tensile strength thereof at around 300.degree. C. is only slightly higher than that of the copper wire. It leads often to the breakdown of a brass wire when the machining speed is raised at such a high temperature with the use of the brass wire. On the other hand, although the stability of electric discharge is enhanced by the presence of a surface zinc layer when a zinc-plated brass wire is used, the strength of such a wire at high temperatures is almost the same as that of the brass wire, so that a breakdown of the wire results if the machining speed is increased. Although tungsten and molybdenum wires have high strengths at high temperatures, they are inferior in adaptability to elongation and too expensive to be used as a consumable electrode wire.
In order to overcome the aforementioned problems, it has been proposed to use an electrode wire composed of a core steel wire coated with a conductive material, such as copper or silver, and further coated with zinc. However, a satisfactory electrode wire having excellent machining properties has not yet been developed.